clean configuration; simple-nixos-mailserver is an ordinary SP module

sp-modules/simple-nixos-mailserver/options.nix

@@ -0,0 +1,13 @@
+{ lib, ... }:
+{
+  options.selfprivacy.modules.simple-nixos-mailserver = {
+    enable = lib.mkOption {
+      default = false;
+      type = with lib.types; nullOr bool;
+    };
+    location = lib.mkOption {
+      default = "sda1";
+      type = with lib.types; nullOr str;
+    };
+  };
+}